The present invention relates to a coated printing sheet. The present invention further relates to methods of manufacturing such a coated printing sheet.
Coated printing papers are typically required to meet many product attribute and performance characteristics. The surface finish, e.g., glossy, dull or matte, and related product quality characteristics of a coated printing sheet are generally dictated by the end use. For example, printed material comprising primarily text is typically printed on paper having a dull or matte finish which facilitates reading; conversely, printed material comprising mostly images, such as magazines, is generally printed on paper having a very glossy finish which tends to accentuate the images.
High quality coated printing papers, regardless of surface finish, are required to meet certain optical properties to ensure that the final printed product exhibits the desired image quality. High quality printing sheets tend to exhibit high brightness which accentuates the color reproduction of the printed images. Because most printing sheets are printed on both sides, the opacity of the printing sheet should be sufficient to reduce show-through of printed text or images from one side of the sheet to the other side.
Other product attributes may affect the performance characteristics of the printing sheet. Smoothness of the sheet tends to enhance the reproduction of images and the clarity of text. Coated printing sheets should exhibit an adequate level of porosity to absorb ink solvents, and in the case of offset lithography, the fountain solution. Coated printing sheets should exhibit a sufficient level of strength and stiffness to withstand printing and any subsequent finishing processes, such as trimming and binding. Printers typically demand printing sheets with relatively high bulk and stiffness to maintain printing press runnability.
Manufacturers of high quality printing sheets generally employ some form of calendering after coating to achieve an appropriate level of paper gloss and smoothness. Increased levels of calendering tend to produce higher gloss and greater smoothness. However, calendering also tends to reduce opacity, stiffness and bulk. Thus, efforts to improve gloss and smoothness through calendering can negatively affect the properties printers desire for runnability.
In addition, high levels of calendering can cause undesirable mottling problems in the paper product and in the final printed image. There are several types of mottling problems, i.e., microgloss and backtrap mottle, which are related to nonuniformities in the paper web. Calendering magnifies these nonuniformities, thereby negatively affecting paper surface quality and final printed image quality. Consequently, paper manufacturers tend to select calendering conditions that optimize certain properties and minimize aesthetically undesirable effects; in doing so other desirable properties are often sacrificed.
Typically, printing papers designed to exhibit low levels of gloss, such as dull or matte grades of paper, are not calendered or are calendered very lightly. Uncalendered printing sheets are particularly susceptible to burnishing, i.e., localized areas of increased gloss or reflectivity on the surface of the sheet typically caused by mechanical rubbing. Uncalendered printing sheets also tend to exhibit greater levels of porosity, which may exacerbate a variety of printing problems if the ink solvent or carrier drains too quickly into the coated surface layer. An overcoat varnish may be applied after printing to protect the paper surface and minimize burnishing but such steps typically add manufacturing complexity and undesirable cost to the final printed product.
To avoid the deleterious effect of calendering, the use of all-latex coatings has been proposed for glossy sheets. Because an all-latex coating tends to form a continuous film on the surface of the sheet, surface gloss tends to be very high. The porosity of such latex coated sheets, however, tends be very low which results in increased ink setting, i.e., the amount of time necessary for the ink on the surface of the coating to dry, or set, sufficiently to allow physical handling, which tends to reduce the quality of the final printed image and to create production inefficiencies. Such glossy sheets also tend to exhibit burnishing.
There remains a need for an offset printing sheet that exhibits the product attributes desired by printers and publishers without the aesthetically undesirable effects discussed above. Specifically there remains a need for an offset printing sheet that exhibits the product attributes typically achieved through calendering without the negative effects of calendering. In addition, there remains a need for an uncalendered printing sheet that does not exhibit the undesirable characteristics of uncalendered sheets, such as burnishing and high porosity.
The inventor has discovered that including a hard polymer pigment having a shear modulus of at least 5.0xc3x97109 dynes/cm2 and a film forming binder in an image receptive coating provides a printing sheet that exhibits the surface and optical properties expected for conventional offset printing grades, and provides a surface that is image receptive and resistant to coating failure or picking, i.e., localized delamination of the coating layer from the underlying substrate, during the manufacturing process and/or during printing. The term xe2x80x9cshear modulus,xe2x80x9d as used herein, means the elastic, or storage, modulus of polymeric material as determined by dynamic mechanical analysis, measured at approximately room temperature, e.g., 21xc2x0 C. The coated printing sheets resist burnishing, i.e., localized areas of increased gloss or reflectivity on the surface of the sheet typically caused by mechanical rubbing. Without intending to be bound by any particular theory, the burnish resistance appears to be related to the resistance to deformation exhibited by the hard polymer pigment particles. Generally, the coated printing sheets exhibit desirable properties, e.g., gloss, bulk, stiffness and smoothness, with minimal or no calendering.
Preferably the image receptive coating of the printing sheets provides sufficient ink drainage or ink setting, i.e., a proportion of the ink solvent carrier drains into the image receptive coating such that the ink on the surface of the coating dries, or sets, sufficiently to allow physical handling of the printed sheet within a relatively short period of time, e.g., 30 to 45 minutes. Ink setting is distinguished from true ink drying which is caused by the complete removal of the solvent and the resulting oxidation of the ink. The coating of the coated printing sheets also exhibits sufficient ink transfer, i.e., absorption of the ink-fountain solution mixture by the image receptive surface is such that a uniform film of ink is transferred from the printing blanket to the sheet during offset printing, and sufficient ink holdout, i.e., the printing ink remains on the surface of the coating. Ink setting, ink transfer and ink holdout affect final product attributes such as the ink gloss and sharpness of the printed image.
In one aspect, the invention provides a printing sheet including a substrate and, on at least one surface of the substrate, an image receptive coating including a film forming binder and a hard polymer pigment having a shear modulus of at least 5.0xc3x97109 dynes/cm2.
Preferred embodiments may include one or more of the following features. The hard polymer pigment has a shear modulus of at least 10.0xc3x97109 dynes/cm2. The hard polymer pigment is essentially non-film forming and remains in the form of discrete roughly spherical solid particles. The hard polymer pigment has a glass transition temperature (Tg) of at least 80xc2x0 C., preferably at least 105xc2x0 C. The hard polymer pigment is selected from the group consisting of poly(methyl methacrylate), poly(2-chloroethyl methacrylate), poly(isopropyl methacrylate), poly(phenyl methacrylate), polyacrylonitrile, polymethacrylonitrile, polycarbonates, polyetheretherketones, polyimides, acetals, polyphenylene sulfides, phenolic resins, melamine resins, urea resins, epoxy resins, and alloys, blends, mixtures and derivatives thereof. The hard polymer pigment has a homogenous composition comprising poly(methyl methacrylate) particles. The hard polymer pigment particles have a particle size of less than about 2,000 angstroms (xc3x85), preferably less than about 1,500 xc3x85, more preferably a particle size ranging from about 600 to 1,200 xc3x85. The image receptive coating includes at least 30 parts by weight of the hard polymer pigment, preferably at least 50 parts, more preferably at least 80 parts, based on 100 parts by weight of total pigment. The term xe2x80x9cparts,xe2x80x9d as used herein, means parts on a dry solids basis, and, as is well known in the art, parts are based on 100 parts of pigment. The film forming binder is selected from the group consisting of latex, starch, polyacrylate salt, polyvinyl alcohol, soy, casein, carboxymethyl cellulose, hydroxymethyl cellulose and mixtures thereof. Preferably the film forming binder is a latex selected from the group consisting of styrene-butadiene, styrene-butadiene-acrylonitrile, styrene-acrylic, styrene-butadiene-acrylic and mixtures thereof. The image receptive coating includes 5 to 75 parts by weight of film forming binder, based on 100 parts by weight of total pigment. The image receptive coating further includes a pigment selected from the group consisting of structured polymer pigment, kaolin, calcined clay, structured clay, ground calcium carbonate, precipitated calcium carbonate, titanium dioxide, aluminum trihydrate, satin white, hollow sphere plastic pigment, solid plastic pigment, silica, zinc oxide, barium sulfate and mixtures thereof. The image receptive coating further includes a structured polymer pigment consisting of a soft domain having a glass transition temperature of less than about 50xc2x0 C. and a hard domain having a glass transition temperature of greater than about 55xc2x0 C. The image receptive coating has a total dried coat weight per side of about 1 to 4 g/m2.
The substrate, prior to application of the image receptive coating, has a smoothness of less than about 3.5 xcexcm, preferably less than about 2.0 xcexcm, more preferably less than about 1.5 xcexcm. The printing sheet further includes at least one precoat layer on the first surface of the substrate underlying the image receptive coating layer. The precoat layer includes a binder and a pigment selected from the group consisting of kaolin, calcined clay, structured clay, ground calcium carbonate, precipitated calcium carbonate, titanium dioxide, aluminum trihydrate, satin white, hollow sphere plastic pigment, solid plastic pigment, silica, zinc oxide, barium sulfate and mixtures thereof. The pigment component of the precoat has a monodisperse particle size distribution. Preferably the monodisperse pigment is selected from the group consisting of precipitated calcium carbonate, hollow sphere plastic pigment and mixtures thereof. The precoat layer has a total dried coat weight per side of about 5 to 15 g/m2.
In another aspect, the invention features a printing sheet including a substrate and, on at least one surface of the substrate, an image receptive coating including a film forming binder and a hard polymer pigment, wherein the hard polymer pigment is essentially non-film forming and remains in the form of discrete roughly spherical solid particles.
In another aspect, the invention provides a method of manufacturing a printing sheet including:
a) applying an image receptive coating, including a hard polymer pigment having a shear modulus of at least 5.0xc3x97109 dynes/cm2 and a film forming binder, to at least a first surface of a substrate; and
b) drying the image receptive coating layer.
Preferred methods may include one or more of the following features. The hard polymer pigment has a shear modulus of at least 10.0xc3x97109 dynes/cm2. A pressing step is performed on the substrate before the application of the image receptive coating at a moisture level ranging from 20 to 60% and at a temperature of at least 100xc2x0 C. A precoating step and a precoat drying step are performed before application of the image receptive coating. A calendering step is performed before the application of the image receptive coating. A calendering step is performed after the image receptive coating drying step, preferably at a nip pressure ranging from 40 to 90 kN/m and a paper surface temperature of at least 5xc2x0 C. lower than the glass transition temperature of the hard polymer pigment. A brushing step is performed after the image receptive coating drying step.
Other features and advantages of the invention will be apparent from the following detailed description, the drawing, and the claims.